Subterranean formations of oil and gas wells are often treated by hydraulically fracturing the formations to increase the production of oil or gas. Fracturing of the formations is accomplished by pumping fluids into the bore holes of the oil or gas wells under high pressure so that cracks or fissures are opened into the surrounding formation. Typically, the fracturing fluid is a polymer which has been gelled to increase its viscosity. The fluid's viscosity is proportionally related to the created fracture geometry and fracture width so that the more viscous fluids will produce longer and wider fractures. After the fracturing fluid is injected into the formation to produce the fracture, the viscosity of the fluid is reduced by means of "gel breakers" which break down the gelled fluid so that it can be easily pumped and removed from the well.
In certain formations, aqueous acid solutions can be used to improve the permeability of the formation, thereby increasing production. These acids are often combined with the polymer gels used in fracturing to provide an acid fracturing fluid. One of the benefits of combining the aqueous acid solutions with gelled fracturing fluids is that the gelled fluid inhibits or retards the reaction of the acid with the formation. This is beneficial in that the acid would otherwise react too quickly, depleting the acid with very little penetration of the formation. Once in place, the viscosity of the fluid is reduced so that the acid is released to react with formation damage or other skin present at the face of the newly formed fractures and improving the permeability of the producing strata.
Crosslinked polymer gels have been particularly useful with these aqueous acid solutions. Crosslinked gels are able to withstand the high temperature conditions commonly found in deeper oil and gas wells with little reduction in viscosity, and they exhibit an improved ability in reducing the reaction rate of the acid solution. Organometallic compounds are often used as a crosslinking agent in these polymer gels. It has been found that gels crosslinked with zirconium and titanium compounds can be treated with certain gel breaking substances, such as fluoride, phosphate or sulfate anions, to break the linkages of the crosslinked polymer fluid, thus reducing the viscosity of the gel.
Typically, crosslinked polymer gels are prepared by batch mixing. In the batch mix process, acid is contained in a storage tank which connects to a blender via a suction pump. The suction pump draws the acid into the blender and through the blender tub, where a polymer (with an inverting agent or internal activator blended in) is added via a chemical additive unit attached to the blender tub. Agitation and shear are applied, and the acid/polymer mixture is circulated back through the storage tank containing the acid. The process is repeated over and over again until an acid gel having the desired viscosity is attained. The acid gel must be stored in the tank to allow the polymer to hydrate before the crosslinker is added. If the crosslinking agent is added too early, it will prevent the hydration of the polymer. Once the polymer is properly hydrated, the crosslinking agent is blended into the acid gel, and the crosslinked polymer gel is injected into the wellbore.
The batch mixing process has several disadvantages, including the delay associated with waiting for the acid gel to re-circulate and waiting for the polymer to hydrate. Typical hydration times with polymers known in the art are 30 minutes to several hours. It is not uncommon for the final polymer gel composition to be of less than desirable consistency, and if polymer concentrates are not hydrated fully, "fish eyes" of unhydrated polymers form. These fish eyes can significantly impair permeability into the wellbore. Batch mixing is also inefficient and costly because if any mechanical problems force the job to shut-down early, the components of the unused batch-mixed gel product must be discarded.
There are also disadvantages associated with polymers known in the art, which are typically available as polymer dispersions with a preblended inverting agent (i.e., an internal activator). Because an inverting agent increases a polymer dispersion's viscosity, a polymer dispersion that has the inverting agent blended in will become more and more viscous over time. As a result, the polymer dispersion's pourability and stability are affected, reducing the polymer dispersion's shelf-life and eventually making the polymer dispersion useless. Further, the end-user cannot control the polymer dispersion's viscosity profile by choosing how much inverting agent to use for a particular application. Finally, the use of nonylphenolethoxylate, an inverting agent commonly preblended in polymer dispersions known in the art, has been banned in the North Sea due to environmental concerns.